Proposition/Definition 2.4 ([1]}, [2]}) 
Let \(F:I=[0,2\pi ]\rightarrow {F}^{\rm sa}\)  be a continuous path. There exists a partition of \(I\) ,
\(0=\theta _0<\theta _1<\ldots <\theta _N=2\pi ,\) 
and corresponding \(a_i>0, i=1,\ldots ,N\) , such that each subpath of spectral projections
\([\theta _{i-1},\theta _i]\ni \theta \mapsto \chi _{[-a_i,a_i]}(F(\theta ))\) 
